The present invention relates generally to cooling apparatuses, and more particularly to a cooling apparatus for cooling an optical element used in an exposure apparatus that exposes an object such as a single crystal substrate for a semiconductor wafer (plate or ball) or a glass plate (wafer) for a liquid crystal display (LCD). The present invention is particularly suitable, for example, for an exposure apparatus that uses ultraviolet light and/or extreme ultraviolet (“EUV”) light as a light source for exposure.
Conventionally, during manufacturing, photolithography technology, a reduction projection exposure apparatus using a projection optical system to project a circuit pattern formed on a mask (reticle) onto a wafer, etc., has been employed for transferring the circuit pattern of fine semiconductor devices such as semiconductor memory and logic circuit.
The minimum critical dimension transferred by the projection exposure apparatus or resolution is proportionate to the wavelength of light used for exposure and inversely proportionate to the numerical aperture (“NA”) of the projection optical system. The shorter the wavelength is, the better the resolution. Thus, along with recent demands for finer semiconductor devices, shorter ultraviolet light wavelengths have been proposed—from an ultra-high pressure mercury lamp (I-line with a wavelength of approximately 365 nm) to KrF excimer laser (with a wavelength of approximately 248 nm) and ArF excimer laser (with a wavelength of approximately 193 nm).
However, lithography using ultraviolet light has limitations when it comes to satisfying the rapidly promoted fine processing of a semiconductor device according to Moore's Law. Therefore, a reduction projection optical system using extreme ultraviolet (“EUV”) light with a wavelength of 10 to 15 nm shorter than that of the ultraviolet (referred to as an “EUV exposure apparatus” hereinafter) has been developed to efficiently transfer very fine circuit patterns of 100 nm or less.
However, because the light absorption in a material increases remarkably as the wavelength of the exposure light becomes shorter, it is difficult to use refraction elements or lens used for visible and ultraviolet light. In addition, no glass material exists for the EUV light's wavelength range. Therefore, reflection-type or catadioptric optical system uses only a reflective element or mirror.
However, the mirror does not reflect all the exposure light, but absorbs about 30% or more. The absorbed exposure light causes residual heat, which deforms the surface shape of the mirror and deteriorates its optical performance, particularly the imaging performance. Therefore, to reduce the mirror's shape change as the temperature changes, the mirror should be made of a low thermal expansion glass, for example, one having a coefficient of linear expansion of 10 ppb.
The EUV exposure apparatus is used for exposure of a circuit pattern of 0.1 μm and has strict critical dimension accuracy requirements. Therefore, the mirror's surface shape is permitted to deform only about 0.1 nm or less. As a result, even a gradual temperature rise on a mirror with a coefficient of linear expansion of 10 ppb would cause the mirror's surface shape to change. For example, a mirror with a thickness of 50 mm has a surface shape change of about 0.1 nm when the temperature rises by 0.2° C. So, for accuracy, the mirror used by an EUV exposure apparatus should keep constant temperature.
However, because the EUV exposure apparatus maintains vacuum atmosphere for an exposure optical path, for example, about 1×10−6 [Pa], so that reaction between the residual gas component in the exposure optical path, such as polymer organic gas, and EUV light does not contaminate the mirror's surface and cause lower reflectance; a heat transfer means to cool the mirror cannot use convection heating such as spraying a gas. It can only perform heat transfer by heat transmission or radiation heat transfer.
The heat conductivity, in general, is very low in the low thermal expansion glass that makes up the mirror. For example, the heat conductivity of the ZERODUR of Shott Co. is 1.6[W/mK]. Therefore, disparate temperature distribution results in the mirror because the mirror cannot properly conduct the absorbed heat to the entire mirror. The disparate temperature distribution causes local deformities (thermal expansion) in the mirror and deteriorates its optical performance. Therefore, in the EUV exposure apparatus, it is necessary to maintain a uniform mirror temperature to avoid generating disparate temperature distribution in the mirror.
FIG. 19 is a schematic structure of a mirror cooling apparatus for using liquid or air as a cooling medium. FIG. 19A is a lower view of the cooling apparatus, whereas FIG. 19B is a side view of the cooling apparatus. In FIG. 19, 1001 is a mirror made of the low thermal expansion glass with low heat conductivity. 1002 is a water pipe for moving water as a cooling medium. 1003 is a joint that connects the water pipe 1002 to a channel 1004 which will be described later. 1004 is the channel for moving the cooling medium into the water pipe 1002. The water pipe 1002 is in contact with a back side of the mirror. The cooling apparatus, by moving the cooling medium into the water pipe 1002, cools the mirror 1001 through heat transfer. As a result, temperature rises on the mirror 1001 can be controlled.
Japanese Laid-Open Patent Application No. 2002-190438 discloses a method of decreasing influence from a heat source. An exposure apparatus exposes a pattern of a mask to a substrate by using a processing part and alignment sensor that processes it, and processing the exposure to a projection optical system. The exposure apparatus comprises a block part that blocks propagation of radiation heat generated from the processing part. Also, Japanese Laid-Open Patent Application No. 05-100096, Japanese Laid-Open Patent Application No. 07-072318, Japanese Laid-Open Patent Application No. 07-174896, Japanese Laid-Open Patent Application No. 08-068897, Japanese Laid-Open Patent Application No. 08-068898, Japanese Laid-Open Patent Application No. 08-211210, Japanese Laid-Open Patent Application No. 08-211211, Japanese Laid-Open Patent Application No. 10-144602, Japanese Laid-Open Patent Application No. 10-339799, Japanese Laid-Open Patent Application No. 11-166691, Japanese Laid-Open Patent Application No. 11-219900, Japanese Laid-Open Patent Application No. 11-243052, Japanese Laid-Open Patent Application No. 11-326598, Japanese Laid-Open Patent Application No. 11-329918, Japanese Laid-Open Patent Application No. 11-345760, Japanese Laid-Open Patent Application No. 2000-098092, Japanese Laid-Open Patent Application No. 2000-100697, Japanese Laid-Open Patent Application No. 2000-286189, Japanese Laid-Open Patent Application No. 2000-349009, Japanese Laid-Open Patent Application No. 2001-215105, Japanese Laid-Open Patent Application No. 2001-241929, Japanese Laid-Open Patent Application No. 2001-280931 and Japanese Laid-Open Patent Application No. 2003-058254 disclose a method for cooling an optical element, in particular, the mirror. However, because it cools the mirror through contact with the mirror, vibrations from the cooling medium's flow through the mirror is transmitted to the mirror, deteriorating the optical performance. Also, because it cools the mirror from the back side, the cooling performance on the mirror is bad. This causes the temperature of the mirror to rise and deform the reflection plane.
The conventional cooling mechanism as described in FIG. 19 cannot completely absorb heat that incidents from an exposure plane of the mirror because it cools the mirror from the back, where heat conductivity is low. Therefore, the heat deforms the mirror, preventing achievement of the intended optical performance.
Also, Japanese Laid-Open Patent Application No. 2002-190438 with only the block part cannot adjust the temperature of the optical element of the projection optical system with high accuracy, causing fear that disparate temperature distribution may occur in the optical element.
In such a case, the cooling medium is directly connected to the mirror's surface and cools as shown in Japanese Laid-Open Patent Application No. 11-243052, Japanese Laid-Open Patent Application No. 2000-286189, Japanese Laid-Open Patent Application No. 2000-349009, Japanese Laid-Open Patent Application No. 2001-215105, Japanese Laid-Open Patent Application No. 2001-241929 and Japanese Laid-Open Patent Application No. 2001-280931. However, the low frequency vibration caused by the flow of the cooling medium to the reflection plane of the mirror is transmitted to the mirror, causing unacceptable influences to the optical performance of the mirror.